Octamethoxydibenzochrysene: Isolation and X-ray crystallographic characterization of a twisted polyaromatic cation radical

Article abstract of DOI:10.1039/b903133b

The isolation and X-ray crystal structure determination of octamethoxydibenzochrysene (3) cation radical together with DFT calculations allow us to delineate evidence that the complex structural changes (i.e. elongation and shortening of various bonds) in

Full text of DOI:10.1039/b903133b

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Octamethoxydibenzochrysene: isolation and X-ray crystallographic
characterization of a twisted polyaromatic cation radicalw
Tushar S. Navale, Linyi Zhai, Sergey V. Lindeman and Rajendra Rathore*
Received (in Austin, TX, USA) 16th February 2009, Accepted 8th March 2009
First published as an Advance Article on the web 6th April 2009
DOI: 10.1039/b903133b
The isolation and X-ray crystal structure determination of
octamethoxydibenzochrysene (3) cation radical together with
DFT calculations allow us to delineate evidence that the complex
structural changes (i.e. elongation and shortening of various
bonds) in a polyaromatic hydrocarbon can be predicted based on
the positioning of the largest bonding and antibonding character
of the HOMO.
The study of polyaromatic hydrocarbons has attracted consid-
erable attention since these molecules hold potential to serve as
building blocks for the preparation of functional electronic and
optoelectronic devices.1 Of these, dibenzochrysene, a twisted
polyaromatic hydrocarbon, and its derivatives have been
explored by Swager and coworkers2 and others3 for the
Scheme 1 Synthesis of octamethoxydibenzochrysene (3) and its
ORTEP diagrams with twisted structure (thermal ellipsoids: 50%
probability).
preparation of sensors, non-linear optical and liquid-crystalline
materials, etc. The aromaticity and structure of parent dibenzo-
chrysene and its dication (formed by 2-electron oxidation) has
been probed both theoretically4 and experimentally,5 however,
structural information is completely lacking.
The electron donor strength of octamethoxydibenzo-
chrysene (3) was evaluated by electrochemical oxidation at a
platinum electrode as a 2 ꢁ 10ꢀ3 M solution in dichloro-
methane containing 0.2 M n-Bu4NPF6 as the supporting
electrolyte. The cyclic voltammograms of 3, if terminated
before the start of the third oxidation event, showed two
reversible oxidation waves (Fig. 1A), which consistently met
Our continuing interest in the design and syntheses of stable
organic cation radicals, or hole carriers, which are of funda-
mental importance to organic materials science,6 prompted the
synthesis of octamethoxydibenzochrysene (3), and isolation
and X-ray crystallographic characterization of its cation-
radical salt. The availability of the X-ray structural data on
the first cationic dibenzochrysene allows us to provide defini-
tive evidence as to how a hole (formed by 1eꢀ oxidation)
induces complex bond length changes in a polyaromatic
hydrocarbon as well as a verification of the experimentally
observed structural changes by DFT calculations. The details
of these preliminary findings are described herein.
the reversibility criteria at various scan rates of 25–500 mV sꢀ1
,
as they all showed cathodic/anodic peak current ratios of
ia/ic = 1.0 (theoretical) as well as the differences between
anodic and cathodic peak potentials of Epa ꢀ Epc E 70 mV at
22 1C (Fig. 1B). The reversible oxidation potentials of 3 were
calibrated with ferrocene as internal standard (Eox = 0.45 V
vs. SCE) and were found to be 0.91 and 1.27 V vs. SCE
corresponding to the formation of monocation and dication,
respectively. It is noted that the third oxidation wave in
the cyclic voltammogram of 3 displays a quasi-reversible
The octamethoxydibenzochrysene (3) was obtained by an
oxidative cyclodehydrogenation of tetrakis(3,4-dimethoxyphenyl)-
ethylene (2) using FeCl3 as an oxidant in a mixture of
dichloromethane and nitromethane in 60% isolated yield.
The tetraveratrylethylene (2), in turn, was prepared from
McMurry coupling of the corresponding tetramethoxybenzo-
phenone7 (1) in 92% yield (see Scheme 1). The structure of 3
was established by 1H/13C NMR spectroscopy and further
confirmed by X-ray crystallography (see Scheme 1 and ESI for
the experimental detailsw).
Department of Chemistry, Marquette University, P. O. Box 1881,
Milwaukee, Wisconsin 53201, USA.
E-mail: rajendra.rathore@marquette.edu; Fax: +1-414-288-7066;
Tel: +1-414-288-2076
w Electronic supplementary information (ESI) available: Synthetic
details of 3 and procedure for the isolation of its cation radical.
CCDC 720096–720097. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b903133b
Fig. 1 (A) Cyclic voltammograms of 2 ꢁ 10ꢀ3 M 3 in CH2Cl2
containing 0.2 M n-Bu4NPF6 at a scan rate of 200 mV sꢀ1 and (B)
cyclic voltammograms of 3 at scan rates of 25–500 mV sꢀ1 at 22 1C.
ꢂc
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oxidation wave (Eox3 = 1.69 V) which, in turn, distorts the
other waves corresponding to the first and second oxidation
events (see Fig. 1A).
example, an excellent crop of dark-colored crystals, suitable
for X-ray crystallographic studies, were obtained by a slow
diffusion of toluene in a dichloromethane solution of 3+
SbCl6ꢀ, obtained using a 1 : 1 mixture of 3 and CRET+
ꢃ
ꢃ
The electrochemical reversibility of 3 and its relatively
low oxidation potential permits its ready oxidation to the
corresponding cation radical using either a hydroquinone ether
SbCl6ꢀ, during a period of 24 h at Bꢀ0 1C.
+
The crystal structure of 3ꢃ SbCl6 revealed that cationic
dibenzochrysene moieties form infinite stacks along the z axis
with non-equivalent interplanar separations of 3.31 and
3.44 A. The robust p–p stacking arrangements amongst the
molecules of the cationic 3 leads to a clathrate structure with
cation radical (CRET+ SbCl6ꢀ; Ered = 1.11 V vs. SCE)8 or
ꢃ
magic blue (MB+ SbCl6ꢀ; Ered = 1.15 V vs. SCE)9 as
ꢃ
oxidants.
Thus, Fig. 2A shows the spectral changes attendant upon
the reduction of blue MB+ (lmax = 728 nm, log e728 = 4.45)
ꢀ
ꢃ
large channels that are filled with SbCl6 counter anions and
by incremental additions of sub-stoichiometric amounts of 3 in
dichloromethane at 22 1C. The presence of multiple isosbestic
points at l = 355, 387, 564, and 783 nm (seeꢃFig. 2A) indicates
a simple electron transfer from 3 to MB+ without decomꢃ-
position of 3+ꢃ. Furthermore, a plot of formation of 3+
(i.e. an increase in the absorbance at 890 nm against the
multiple dichloromethane molecules (see Fig. 3A/B). In
contrast, the packing in the crystal structure of neutral 3 is
dominated not by p–p but by CHꢄ ꢄ ꢄp interactions amongst the
methoxy groups and the electron-rich aromatic rings of
the neighboring molecules. The resulting honeycomb-like
layers, formed perpendicular to the crystallographic x-axis, are
interspaced by disordered dichloromethane molecules
(see Fig. 3D); and the layers are separated by copious amounts
of acetonitrile molecules (see Fig. 3C).z
increments of added 3, Fig. 2B) established that MB+ was
ꢃ
completely consumed after the addition of 1 equiv. of 3; and
the resulting highly structured absorption spectrum of the 3+
ꢃ
[with intense absorption bands at lmax = 890 (log e890
4.37), 513, 458, and 394 nm and relatively weak bands at
max = 636 (log e634 = 3.67) and 784 nm] remained unchanged
upon further addition of neutral 3, i.e., eqn (1).
=
An inspection of the bond length changes in the cation
radical 3+ꢃ, together with a comparison of its neutral form,
points to the following salient features. (i) Neutral 3 has a
crystallographic 2-fold symmetry and its planarity is
l
3 + MB+ - 3+ + MB
(1)
ꢃ
ꢃ
It is noted that although an identical spectrum of 3+ was
ꢃ
obtained when CRET+ was treated with an equimolar
ꢃ
amount of 3, a clean spectral titration plot with isosbestic
point could not be obtained owing toꢃan overwhelming over-
lapꢃof the absorption band of CRET+ at 518 nm with that of
3+ (see ESIw). It is further noted that the intensely colored
solution of 3+ꢃ, obtained according to eqn (1) or using
CRET+ꢃ, was stable at ambient temperature and did not
show any decomposition during a 48 h period at 22 1C, as
confirmed by UV-Vis spectroscopy. Moreover, a reduction of
3+ with zinc dust regenerated the neutral 3 quantitatively as
ꢃ
1
confirmed by H NMR spectroscopy.
The high stability of 3+ SbCl6ꢀ in solution prompted us to
ꢃ
attempt the isolation of its crystalline salt as follows. For
+
ꢀ
Fig. 3 (A) The packing diagram of 3ꢃ SbCl6 showing that the
+
ꢀ
channels formed by stacked 3ꢃ are filled with SbCl6 and CH2Cl2
Fig. 2 (A) The spectral changes observed upon the reduction of 3.6 ꢁ
molecules.ꢀ(B) Space filling representation of the packing diagram of
+
ꢀ
10ꢀ5 M MB+ by an incremental addition of substoichiometric
3ꢃ SbCl6 where one of the channels is shown without SbCl6 and
CH2Cl2 molecules. (C) The packing diagram of neutral 3 showing the
layered structure where the layers are separated by acetonitrile mole-
cules. (D) The arrangement of 3 with embedded CH2Cl2 molecules
within a single layer of neutral 3.
ꢃ
amounts of 3 in CH2Cl2 at 22 1C. (B) A plot of depletion of
absorbance of MB+ (blue triangles, at 728 nm) and an increase of
ꢃ
the absorbance of 3ꢃ+ (red circles, at 890 nm) against the equivalents
of added neutral 3.
ꢂc
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Table 1 Experimental and theoretical bond lengths of the neutral and
cation radical of 3 presented in picometres (pm). Numbering scheme
for the skeleton of 3 and its HOMO, obtained by DFT calculations at
B3LYP/6-31G* level
In summary, octamethoxydibenzochrysene (3) is easily
accessed from readily available starting materials and it under-
goes reversible electrochemical oxidation and forms a highly
robust cation-radical salt. The X-ray crystal structure deter-
mination of 3+ꢃ SbCl6ꢀ as well as neutral 3 together with DFT
calculations provides unequivocal evidence that an intro-
duction of a cationic charge (or polaron) in polyaromatic
hydrocarbon 3 leads to a complex elongation and shortening
of the various bonds. The observed bond length changes in
3+ can be readily reconciled by the positioning of the largest
ꢃ
bonding and antibonding character of the HOMO in neutral
3. The close packing of the molecules of the cationic 3 in the
crystals produces large channels akin to those found in zeolites
and may allow the preparation of potentially useful conduct-
ing materials by utilizing electro-active counter anions.11
We thank the National Science Foundation (CAREER
Award) for financial support.
B3LYP/6-31G*
X-Ray data
Bonda
3
3+
ꢃ
D
3
3+
D
ꢃ
A
B
C
D
E
F
G
H
I
140.3
145.8
142.2
138.1
142.4
138.1
141.7
141.7
145.2
136.3
141.6
136.2
141.6
—
144.0
144.1
142.4
138.0
143.0
139.3
140.4
142.2
146.3
135.1
142.3
134.1
142.7
—
+3.7
ꢀ1.7
+0.2
ꢀ0.1
+0.6
+1.2
ꢀ1.3
+0.5
+1.1
ꢀ1.2
+0.7
ꢀ2.1
+1.1
—
141.4
145.5
141.5
136.9
141.9
137.8
142.0
141.6
145.3
137.0
143.3
136.3
142.9
0.5
143.0
143.9
142.3
136.9
142.5
138.2
140.1
141.6
146.5
135.8
143.2
134.9
144.2
0.4
+1.6
ꢀ1.4
+0.8
0.0
+0.6
+0.4
ꢀ1.9
0.0
+1.2
ꢀ1.2
ꢀ0.1
ꢀ1.4
+1.3
—
Notes and references
z Crystal structure data for 3 [C34H32O8ꢄCH2Cl2ꢄ(CH3CN)4] (raj2z):
FW = 817.74, monoclinic, C2/c, a = 31.6750(8) A, b = 7.3983(2) A,
c = 18.0078(4) A, b = 106.7180(10)1, Z = 4, V = 4041.60(17) A3,
D = 1.344 Mg mꢀ3, T = 100 K, 6231 reflections measured, 3114
unique reflections, Rint = 0.0174, 342 parameters refined, R(all) =
0.0944, wR(all) = 0.2397, S = 1.066 (CCDC 720097). Crystal
J
K
L
M
s
+
structure data for 3ꢃ SbCl6ꢀ[C34H32O8SbCl6ꢄ(CH2Cl2)2] (raj3d):
ꢀ
FW = 1072.90, triclinic, P1, a = 13.6451(5) A, b = 14.0770(5) A,
a
Average of equivalent bonds.
c = 14.3621(5) A, a = 61.672(2)1, b = 62.275(2)1, g = 70.899(2)1,
Z = 2, V = 2129.39(13) A3, d = 1.673 Mg m‘3, T = 100 K, 16984
reflections measured, 6159 unique reflections, Rint = 0.0343, 644
parameters refined, R(all) = 0.0307, wR(all) = 0.0.0728, S = 1.039
(CCDC 720096).
substantially distorted by twisting around the central CQC
bond (denoted as A, see Table 1) by 25.81 and around the
central bonds of ‘‘biphenyl’’ fragments (denoted by I) by 11.41.
One electron oxidation of 3 results only in a minor amplifica-
tion of the distortions as judged by the slightly increased twist
angles of 29.1 and 11.91 for bonds A and I, respectively. (ii) As
in various other aryl-methyl ether catioꢃn radicals,10 O–C(ar)
bonds (denoted as J and L) in 3+ exhibit shortening
by B1.3 pm due to an increased p–p dative interaction. (iii)
Although the rearrangement of the lengths of various bonds
(i.eꢃ. elongation and shortening) in the polyaromatic moiety in
3+ has a complex character (see Table 1), the changes clearly
correspond to the predominant contributions from the
resonance structures I/II, as judged by the significant length-
ening of bonds A, C, F, I, and M and shortening of bonds B,
G, J, and L and only a minor contribution from the resonance
structures III/IV (see below).
1 (a) J. Wu, W. Pisula and K. Muellen, Chem. Rev., 2007, 107, 718;
(b) A. P. H. J. Schenning and E. W. Meijer, Chem. Commun., 2005,
3245.
2 (a) J.-S. Yang and T. M. Swager, J. Am. Chem. Soc., 1998, 120,
5321; (b) S. Yamaguchi and T. M. Swager, J. Am. Chem. Soc.,
2001, 123, 12087; (c) A. Rose, J. D. Tovar, S. Yamaguchi,
E. E. Nesterov, Z. Zhu and T. M. Swager, Philos. Trans. R. Soc.
London, Ser. A, 2007, 365, 1589, and references therein.
3 (a) S. Kumar and S. K. Varshney, Mol. Cryst. Liq. Cryst. Sci.
Technol., Sect. A, 2002, 378, 59; (b) C.-W. Li, C.-I. Wang,
H.-Y. Liao, R. Chaudhuri and R.-S. Liu, J. Org. Chem., 2007,
72, 9203; (c) V. M. Vincent and J. D. Wright, J. Chem. Soc. Perkin
Trans. 1, 1974, 58; (d) S. J. Toal and W. C. Trogler, J. Mater.
Chem., 2006, 16, 2871.
4 (a) J.-I. Aihara, J. Phys. Org. Chem., 2008, 21, 79; (b) N. S. Mills,
J. Org. Chem., 1992, 57, 1899.
5 D. A. Forsyth and G. A. Olah, J. Am. Chem. Soc., 1976, 98, 4086.
6 (a) M. Banerjee, S. V. Lindeman and R. Rathore, J. Am. Chem.
Soc., 2007, 129, 8070; (b) J. K. Kochi, R. Rathore and
P. L. Magueres, J. Org. Chem., 2000, 65, 6826; (c) R. Rathore,
S. H. Abdelwahed and I. A. Guzei, J. Am. Chem. Soc., 2004, 126,
13582; (d) P. Debroy, R. Shukla, S. V. Lindeman and R. Rathore,
J. Org. Chem., 2007, 72, 1765, and references therein.
7 M. A. Silvestri, M. Nagarajan, E. De Clercq, C. Pannecouque and
M. Cushman, J. Med. Chem., 2004, 47, 3149.
8 R. Rathore, C. L. Burns and M. I. Deselnicu, Org. Synth., 2005,
82, 1.
9 (a) F. A. Bell, A. Ledwith and D. C. Sherrington, J. Chem. Soc. C,
1969, 13, 2719; (b) F. A. Bell, A. Ledwith and D. C. Sherrington,
J. Org. Chem., 1976, 13, 155.
10 (a) D. Sun, S. V. Lindeman, R. Rathore and J. K. Kochi, J. Chem.
Soc., Perkin Trans. 2, 2001, 1585; (b) R. Rathore, S. V. Lindeman,
A. S. Kumar and J. K. Kochi, J. Am. Chem. Soc., 1998, 120,
6931–6939.
11 Compare: P. L. Magueres, S. M. Hubig, P. Veya and J. K. Kochi,
J. Am. Chem. Soc., 2000, 122, 10073.
The experimental observations of the bond length changes
in 3+ were found to be in reasonable agreement with the
ꢃ
calculated values using DFT calculations at the B3LYP/
6-31G* level (see Table 1). Furthermore, the experimentally
observed elongation and shortening of the bonds in 3+
tracked remarkably well with the positioning of the largest
ꢃ
bonding and antibonding character of HOMO in
(see Table 1).
3
ꢂc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 2857–2859 | 2859